The present invention relates to a directivity control apparatus for an adaptive antenna used in a radio station such as a radio base station.
A radio station such as a radio base station which communicates with a radio terminal such as a portable telephone uses an antenna for radiating radio waves. In general, radio waves radiated by the antenna propagate over the range of cells assigned to respective radio base stations. Radiating radio waves in a direction other than the direction of a radio terminal consumes power supplied to the antenna in transmitting radio waves.
Radio waves are generally radiated strongly in a direction in which a radio terminal exists, and weakly in a direction in which no radio terminal exists. An example of an antenna capable of adjusting the directivity of radio waves is an adaptive antenna having a directivity control apparatus.
There has conventionally been proposed a technique of installing an adaptive antenna in a radio base station and adjusting the transmission directivity of a signal transmitted from the adaptive antenna in accordance with the position of a moving radio terminal. In Japanese Patent Laid-Open No. 11-298400 (reference 1), a predetermined signal is transmitted to a radio terminal while the directivity of radio waves is changed in a radio base station. The directivity at which the reception power of a predetermined signal received by the radio terminal is strongest is used as the transmission directivity. A similar arrangement is also disclosed in Japanese Patent Laid-Open No. 09-200115 (reference 2).
In Japanese Patent Laid-Open No. 10-070502 (reference 3), the arrival direction of a reception signal which has arrived at a radio base station is estimated by calculation. The transmission directivity is so controlled as to radiate a transmission signal strongly in the estimated direction.
When a plurality of radio terminals exist within the cells of a radio base station, the radio base station simultaneously radiates radio waves to the radio terminals. Particularly if there are a plurality of radio terminals which perform communication by signals of the same or neighboring frequencies, radiated radio waves may interfere with each other, failing to ensure proper communication. As a technique of suppressing interference between radio waves, the adaptive antenna also receives a great deal of attention.
A radio base station using the adaptive antenna comprises a transmission directivity control apparatus for uniquely weighting outputs to a plurality of antenna elements which constitute an antenna, and adjusting the transmission directivity. To generate the transmission directivity pattern of a radio terminal, a main beam in a direction in which radio waves most strongly reach the radio terminal is directed. In addition, a null in a direction in which radio waves weakly reach other radio terminals is directed. The beam and null of the transmission directivity pattern can be directed to desired directions by adjusting antenna weights representing the weighting coefficients of outputs to respective antenna elements. Desired transmission directivity patterns are generated for respective radio terminals. This can suppress interference with radio waves propagating to other radio terminals to an extent that the nulls of respective transmission directivity patterns are directed even if radio waves are simultaneously radiated in transmission.
FIG. 18 shows a conventional transmission directivity control apparatus in a radio base station using an adaptive antenna. In the prior art, channels are assigned to first to Mth radio terminals (not shown) for the same time interval in the same frequency band by using CDMA (Code Division Multiple Access). In FIG. 17, a transmission directivity control apparatus 11 comprises first to Nth antenna elements 121 to 12N which are regularly aligned at an equal interval and transmit/receive radio waves. The antenna elements 121 to 12N are respectively connected to first to Nth transmission/reception demultiplexing circuits 131 to 13N for demultiplexing reception and transmission signals of radio waves. The transmission/reception demultiplexing circuits 131 to 13N are connected to a reception unit 14 for receiving reception signals, and a transmission unit 15 for transmitting transmission signals.
The reception unit 14 is constituted by first to Nth receivers 161 to 16N for amplifying and detecting reception signals output from the transmission/reception demultiplexing circuits 131 to 13N, and first to Nth A/D (analog-to-digital) converters 171 to 17N for converting amplified/detected reception signals into digital signals. The transmission unit 15 is constituted by first to Nth D/A (digital-to-analog) converters 181 to 18N for converting transmission signals into analog signals, and first to Nth transmitters 191 to 19N for modulating and amplifying transmission signals converted into analog signals, and outputting the modulated/amplified signals to the transmission/reception demultiplexing circuits 131 to 13N. The reception unit 14 and transmission unit 15 are connected to first to Mth transmission directivity generation units 201 to 20M for generating transmission directivities assigned to respective first to Mth radio terminals by the radio base station.
The first transmission directivity generation unit 201 has a reception directivity pattern control unit 211 for receiving reception signals converted into digital signals by the A/D converters 171 to 17N and generating a reception directivity pattern. The output of the reception directivity pattern control unit 211 is connected to an antenna weight correction unit 221 for correcting various errors when each antenna weight calculated in generating a reception directivity pattern is used to generate a transmission directivity pattern. The output of the antenna weight correction unit 221 is connected to a transmission directivity pattern control unit 231 for receiving each antenna weight obtained by correcting various errors, and generating a transmission directivity pattern. The output of the transmission directivity pattern control unit 231 is connected to the D/A converters 181 to 18N for receiving transmission signals weighted by antenna weights calculated in generating a transmission directivity pattern.
Similar to the first transmission directivity generation unit 201, the second to Mth transmission directivity generation units 202 to 20M comprise reception directivity pattern control units 212 to 21M, antenna weight correction units 222 to 22M, and transmission directivity pattern control units 232 to 23M. The same arrangement as that of the transmission directivity control apparatus is also disclosed in Japanese Patent Laid-Open No. 2000-209017 (reference 4). The operations of the transmission directivity generation units 201 to 20M are the same, and the operation of the transmission directivity generation unit 201 will be representatively explained.
The reception directivity pattern control unit 211 receives the reception signals of the antenna elements 121 to 12N which have simultaneously received radio waves from the first to Mth radio terminals. The reception directivity pattern control unit 211 executes despreading of multiplying the reception signals by a spreading code multiplied in the first radio terminal, and separates the reception signals of the first radio terminal from the remaining spread reception signals. The reception directivity pattern control unit 211 calculates antenna weights corresponding to the reception signals from the first radio terminal. The reception directivity pattern control unit 211 weights the reception signals by the antenna weights to generate a reception directivity pattern for the first radio terminal.
The antenna weight correction unit 221 receives the respective antenna weights calculated by the reception directivity pattern control unit 211. When the frequencies of reception and transmission signals are different from each other, the antenna weight correction unit 221 corrects an antenna weight error caused by the frequency difference between the reception and transmission signals. At the same time, the antenna weight correction unit 221 corrects antenna weight errors caused by amplitude and phase deviations generated in the reception unit 14 and transmission unit 15.
The transmission directivity pattern control unit 231 weights a transmission signal 241 from a transmission signal generation unit (not shown) by each corrected antenna weight to generate a transmission directivity pattern. The transmission directivity pattern control unit 231 performs spreading of multiplying the transmission signal by a spreading code. The spread transmission signal is input to the D/A converters 181 to 18N.
The transmission directivity pattern generated by the transmission directivity control apparatus 11 is generated using substantially the same antenna weight as each antenna weight calculated in generating a reception directivity pattern. The transmission directivity pattern is substantially the same as the reception directivity pattern. For example, when the reception directivity pattern control units 211 to 21M perform MMSE (Minimum Mean Square Error) adaptive control as a method of calculating each antenna weight, each antennal weight which directs a null to the direction of a large-reception-power signal is generated. A transmission directivity pattern generated in the transmission directivity control apparatus 11 directs a null to the direction of the large-reception-power signal. Since interference with radio waves from another radio terminal is small in the direction to which the null is directed, necessary transmission power to a radio terminal in this direction is decreased. As a result, interference with another radio terminal can be reduced.
In recent years, various contents obtained by downloading various data such as image data on the Internet from a radio terminal such as a portable telephone have increasingly being used. In this data communication, a downlink signal tends to have a larger capacity of data than an uplink signal, and higher data transmission rate is being required more and more. In general, the transmission power of radio waves becomes larger for higher data transmission rate. If the data transmission rates of uplink and downlink signals are different, the power distributions of the uplink and downlink signals are also different.
The conventional radio base station uses a reception directivity pattern optimized for the power distribution of the uplink signal as a transmission directivity pattern on the assumption that the transmission rates of the uplink and downlink signals are the same. Hence, the adaptive antenna effects cannot be fully enhanced to suppress interference.
To solve this problem, there is proposed a technique of making the direction to which the null is directed correspond to the power distribution of the downlink signal. In Japanese Patent Laid-Open No. 2000-224097 (reference 5), the null is directed to the large-transmission-power direction of a downlink signal. This reduces necessary transmission power to a radio terminal in this direction, and decreases interference in the small-transmission-power direction of another downlink signal.
The number of nulls which can be adjusted by the transmission directivity control apparatus is restricted by the number of antenna elements. When a plurality of radio terminals exist in addition to a radio terminal to which the main beam is directed, nulls may not be able to be directed toward all the remaining radio terminals. In some cases, the number of nulls exceeds the restricted number of nulls due to an increase in radio terminals which require data communication. In this case, nulls cannot be directed to radio terminals whose downlink signal transmission powers are large. In this case, large transmission power to radio terminals in directions to which no null is directed cannot be reduced. Radio interference with other small-transmission-power radio terminals cannot be suppressed.
The relationship between the main beam and the null has been described. Similar problems also occur in another case. When the directivity pattern is adjusted such that the radio intensity increases for target transmission radio terminals and decreases for other time-overlapping radio terminals, a plurality of radio terminals cannot be selected as other radio terminals to be selected.